1. Field of the Invention
The present invention relates to a high voltage direct current system (abbreviated as “HVDC” system hereinafter) and more particularly, to a method for calculating an electric power value using values measured through a voltage sensor and an electric current sensor.
2. Description of the Conventional Art
The HVDC system means an electric power transmission system that an electric power transmission station converts an alternating current (AC) electric power generated by an electric power station to a direct current (DC) electric power to transmit the electric power and then an electric power receiving station reconverts the DC electric power to the AC electric power to supply the electric power.
The HVDC system is applied to undersea cable electric power transmission, large capacity long-distance electric power transmission, AC grid interconnection, etc. Also, the HVDC system enables grid interconnection of different frequencies and asynchronism interconnection.
The electric power transmission station converts an AC electric power to a DC electric power. That is, since the status that the AC electric power is transmitted using an undersea cable, etc. is very dangerous, the electric power transmission station converts the AC electric power to the DC electric power and then transmits the electric power to the electric power receiving station.
Meanwhile, various kinds of voltage type converters are used for the HVDC system, and a modular multi-level converter has recently received much attention.
The modular multi-level converter (MMC) is a device that converts an AC electric power to a DC electric power by using a plurality of sub-modules, and is operated by controlling each sub-module in a state of charge, discharge and bypass.
Potential transformers are installed in several sites of the HDVC system in order to control and protect the system.
However, the potential transformer may sense a different measuring value depending on a range of voltage measuring even with the same device and therefore measuring errors might occur, which might be recognized as a system failure, whereby system operation might have to be stopped in a serious condition.
That is, voltage measuring errors of a single potential transformer should have a margin between 0.2% and 0.5%, in general. However, the potential transformer installed in the HDVC system has a very big margin of measuring errors when measuring extra-high voltage, also in the case where a plurality of potential transformers are installed in several sites, the margin of errors caused by a measuring error of each potential transformer will have no option but to get wider.
Also, a more precise sensing device can be used to solve this problem but using a more precise sensing device will bring about installation cost and some technical problems.